Interim oxidation of silsesquioxane dielectric for dual damascene process

ABSTRACT

Resist developers can attack some advanced dielectric materials such as silsesquioxane materials which can be used as an insulator between a surface of an integrated circuit chip and wiring layers formed on the surface of the dielectric material. By performing a resist stripping or etching process in which a reactant material is supplied externally or liberated from the dielectric material, an extremely thin surface protective covering of an intermediate material may be formed which is impervious to resist developers or any of a plurality of other materials which may damage the flowable oxide material. A dual Damascene process for forming robust connections and vias to the chip can thus be made compatible with advanced dielectrics having particularly low dielectric constants to minimize conductor capacitance and support fast signal propagation and noise immunity even where conductors are closely spaced to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to integrated circuitmanufacturing processes and, more particularly, to processes for formingrobust conductive structures on advanced insulating materials.

2. Description of the Prior Art

Increases in both performance and manufacturing economy with increasedintegration density have led to the use of design rules with extremelysmall feature sizes and closeness of spacing in integrated circuitdesign. Reduced interconnection length between devices included inintegrated circuits generally reduces signal propagation time andincreases noise immunity. However, as integrated circuits are scaled tosmaller design rules, reductions in wiring delays are diminished byincreases in resistance (due to decreased cross-sectional area) andcapacitance (due to increased connection proximity). This effect canonly be ameliorated by reduced resistivity of conductors and/ordielectric constant of insulators. Accordingly, advanced dielectricmaterials having low dielectric constants have been developed and usedin manufacture of integrated circuits.

Among such advanced materials having particularly low dielectricconstants now in widespread use are some silsesquioxane materials(hereinafter SSQ) which are particularly preferred for their ease ofapplication, much in the nature of conventionally applied spin-on glassand gap filling qualities. The filling of gaps is important to reducelikelihood of propagation of crystal lattice dislocations and cracks insemiconductor materials which may carry significant internal stresses.

Suitable SSQ materials are commercially available polymer materials, oneof which is principally, if not entirely, hydrogen silsesquioxane(HSiO_(3/2)), sometimes referred by the acronym HSSQ or HSQ. A relatedmaterial sometimes employed is methyl silsesquioxane (MSSQ or MSQ) inwhich a methyl group is substituted for the hydrogen in HSQ. Thesematerials form a structure in which bridging oxygen atoms are sharedbetween molecules. While the dielectric constant is low, the bridgingoxygen atoms are strained and can be attacked by numerous chemicals orresult in rapidly propagating cracks from relatively minor physicaldamage. For that reason, unprotected SSQ materials are not generallyconsidered suitable for formation of layers even though they exhibitgood planarizing characteristics that would support high resolutionlithographic processes. For example, U.S. Pat. No. 5,818,111 notes therelative fragility of HSQ and proposes a structure of alternating layersof HSQ and protective silicon dioxide to form a multi-layer dielectricstack.

Damascene processing, alluded to above, is a well-understood and maturetechnology which is particularly useful for mechanically forming robustconnections of superior electrical integrity at very small sizes andclose spacings. Basically, a Damascene process defines the desired shapeof conductors by the formation of a groove or recess in the surface of adielectric material followed by deposition of a layer of metal ofsufficient thickness to fill the recesses. The layer of metal is thenreadily patterned by planarization to the original surface of thedielectric by any known process such as polishing. The structure soformed fully supports the metal at the bottom and sides of a connection(which may be made of materials such as gold, aluminum, tungsten orcopper) and thus is resistant to metal migration, damage or the like.The formation of the groove or recess can also generally be formed withgreater precision and regularity of the edges of the pattern than can beachieved by direct patterning of the layer of metal.

When forming conductors on a dielectric layer above a substantiallycompleted chip, however, two patterning processes are required forformation of the interconnect patterns and for forming connections todevices on the chip in the form of vias alluded to above, respectively.These two patterning processes followed by metal deposition andplanarization are collectively referred to as a dual Damascene process.

However, most SSQ materials are readily attacked by most lithographicresist developers which are generally of high pH. Moreover, when the SSQmaterial is attacked by resist developers, the amount of material whichmay be removed is not readily controllable and may undercut the resistpattern. Thus, when the SSQ is etched, even with a well-controlledetchant, the resulting via shape may be quite distorted and vias willlack uniformity across the chip and possibly be irregularly andunreliably registered with the structures on the chip to which the viasare to form connections.

Protection of the surface of the SSQ material with a further layer ofmaterial such as another dielectric is not practical in somesemiconductor structures such as formation of connection vias in a dualDamascene process. Virtually any otherwise suitable material forprotection of an SSQ material would have a dielectric constant which ishigher than that of SSQ and even a very thin layer would increasecapacitance at a location where capacitance may be critical and maypossibly require a different etchant and/or an additional etchingprocess to remove in accordance with a resist pattern. Additionally,while a protective layer may be deposited on an original surface of anSSQ layer deposition of a protective layer in an etched feature (e.g. ina trench or groove) would require additional process steps as well ascompromising the low capacitance of any conductive structure formedtherein for which the SSQ material was employed.

Of course, the concept of using a resist is predicated on the assumptionthat the underlying material will not be affected by the processing anddevelopment of the resist until the resist pattern is fully formed.Since the SSQ material removal by the resist developer appears to be afunction of the breaking of fragile shared oxygen bonds, it is unlikelythat a developer could be found which would not attack the SSQ material.Accordingly, at the present state of the art, processes using multipleresist layers would provide no significant benefit toward reduction ofthe problem.

In summary, use of HSQ or MSQ to underlie or support interconnectionmetallization allows use of dual Damascene processes to produceconductors and vias only with the likelihood that manufacturing yieldwill be compromised. While SSQ materials can be used as a gap fillmaterial over connections applied to a surface, the advantages ofdamascene connections noted above are not achieved thereby. No techniquehas been available for avoiding the basic incompatibility of SSQmaterials and resist developers in processes requiring a sequence ofpatterned etch steps, such as in a dual damascene process, particularlyfor accommodating fine pitch design rules for high density integratedcircuits where the low dielectric constant of SSQ materials isparticularly critical.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a processfor protection of SSQ dielectric materials from attack by developingsolutions during resist patterning without deposition of an additionalprotective layer for that purpose in, for example, an etched featurewhere an initially provided protective film would have been removed.

It is another object of the invention to provide a process by whichrobust interconnections between devices on a chip can be formed by adual Damascene process consistent with use of SSQ dielectric materialsand high manufacturing yield.

It is a further object of the invention to provide a process of formingdual Damascene conductors of reduced cost.

It is yet another object of the invention to provide protection for aSSQ dielectric material during dual Damascene processing as an incidentof other required steps to avoid additional process steps (andvariation) and costs associated therewith.

In order to accomplish these and other objects of the invention, amethod and semiconductor device formed thereby are provided by the stepsof forming a pattern of resist on a surface of a material, removing aportion of the material in accordance with the pattern of resist whileforming an intermediate material composition which is substantiallyimpervious to a selected material at an exposed surface of the materialas material is removed, forming a further pattern of resist on a surfaceof the intermediate material, and removing a portion of the intermediatematerial and a further portion of the material in accordance with thefurther pattern of resist.

In accordance with another aspect of the invention, a semiconductordevice is provided including, in combination, a layer of silsesquioxanematerial, a damascene conductor formed in a groove in a surface of thesilsesquioxane material, a via extending through the silsesquioxanematerial and located within said groove below the damascene conductor,and an oxidized surface on the silsesquioxane material within thegroove.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1 and 2 are cross-sectional views of initial stages of a dualDamascene process of a portion of an integrated circuit chip,

FIG. 3 is a cross-sectional view of via resist processing on a portionof an integrated circuit chip in a dual Damascene process illustratingeffects of resist development without the present invention,

FIGS. 4 and 5 are cross-sectional views of a portion of an integratedcircuit chip illustrating the practice of the invention and theprotection of the SSQ dielectric achieved thereby, and

FIG. 6 is a cross-sectional view of the completed Damascene conductorand via in accordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown in cross-section, an early stage in dual Damascene processing toproduce conductors at a surface of structure 10 including an integratedcircuit chip 12 having devices such as transistors, capacitors andunderlying connections (not shown) formed thereon, an SSQ dielectriclayer 14, a thin oxide dielectric layer 16 and a patterned resist layer18. It should be understood that FIGS. 1-3 are not to scale and aredisproportionately enlarged in some regions in the interest of clarity.It is also to be understood that while FIGS. 1 and 2 depict early stagesof a well-understood dual Damascene process and FIG. 3 is illustrativeof the effects of resist development in the absence of the invention, noportion of FIGS. 1-3 is admitted to be prior art as to the presentinvention.

The illustration of FIG. 1 assumes prior deposition, processing (e.g.drying), exposure and development of resist layer 18 to form an aperture20 at the location of a connection to be formed. The resist and themetal layer illustrated, together with SSQ layer 14 and thin oxide layer16 should be understood as being representative of any layer at anylevel and a plurality of such grouping of layers (e.g. 14 and 16) can beformed sequentially over other such layers and patterned in turn, atwill, to develop desired connection patterns.

As shown in FIG. 1, development of the resist does not affect the SSQlayer 14 due to the protection provided by thin oxide layer 16. Thethickness of the thin oxide is designed to cause minimal increase incapacitance in the final structure and can potentially be removed.However, it should be appreciated that the resist developer cannotattack the SSQ dielectric due to the presence of the thin oxide 16 whichprevents the resist developer from reaching SSQ layer 14.

As shown in FIG. 2, an etching process is carried out to form anaperture 22′ in thin oxide layer 16 and a recess 22 in SSQ layer 14 inaccordance with the pattern 20 developed in resist 18. It should benoted that the etching process is reasonably well-behaved andanisotropic; following the patterning 20 of resist layer 18. Numerousprocesses providing results substantially as depicted are known in theart. The remainder of resist layer 18 can then be stripped away asdepicted by dashed outlines of the profile thereof.

If only a conductor was to be formed by a single damascene process, ablanket metal layer could be deposited over all exposed surfaces andpatterned by polishing, isotropic etching or the like to or through thinoxide layer 16 and no problem would arise. The problem addressed by theinvention arises from the fact that portions of such a conductor must beconnected to electronic structures on chip 12. The SSQ layer must thusbe further patterned to form vias in order to do so when the metalblanket layer is deposited in accordance with a dual damascene process,as will now be explained with reference to FIG. 3. (Low resistancemetals such as copper are not easily etched in a manner compatible withetching of SSQ materials and it would therefore be difficult to modifythe dual damascene process to deposit the conductor to be used as a maskfor the SSQ materials while forming the vias. The aspect ratio of thevias would also be increased by such a process, compromising the abilityto fill the vias with metal.)

As shown in FIG. 3, another resist layer 24 is applied and exposed anddeveloped to form pattern 26 which is intended to define the locationsof vias which will be etched to reach structures formed on chip 12 inmuch the same manner as described above in connection with FIG. 2.However, the process of development of the resist progressivelydissolves the resist into the developer and must be carried tocompletion to dissolve substantially all resist which is soluble in thedeveloper in accordance with the patterning exposure.

Completion of the development process to properly form apertures 26 inthe via resist 24 thus makes exposure of the surface 28 of the SSQ layerunavoidable (in the absence of the invention) for a somewhat extendedperiod of time. As noted above, the developer can attack the SSQmaterial and does so isotropically. The developer thus removes materialand forms a cavity or reacted zone that can extend under the resist andcompromises both the integrity of the resist and the ability of theresist to properly control a subsequent etching process to form via 25in the location and size intended.

That is, even a very well-controlled anisotropic etching process carriedout after undercutting 28′ of the via resist 24 had occurred, would belikely to form a via of the shape and location indicated at 27 ratherthan 25. As can be seen, via 27 is larger than intended, and is ofirregular shape with its center shifted from that of via 25. Therefore,registration of the via with structures on chip 12 is compromised and,moreover, the increased size and potential error in location can causeshorting of structures on the chip.

Referring now to FIG. 4, the inventors have discovered that performingthe etching described above with reference to FIG. 2 with certainpreferred etchants and preferably using a reactive ion etching process,an extremely thin protective oxide film 40 can be provided on thesurface of the SSQ material as it is exposed by the etching process.That is, an oxide-producing reaction is induced as part of the etchingprocess which is self-limited to the exposed silicon atoms (and,possibly, silicon atom reachable by diffusion for a few hundredAngstroms ahead of the material being etched away) in the SSQ polymer.

This self-limited thickness of protective oxide is sufficiently smallthat the low dielectric constant of the SSQ material is not compromisedsignificantly. The oxide can be reliably formed in a manner thatprotects the surface of the SSQ material due to the fact that it is asurface effect which involves all exposed silicon atoms. The etching ofthe oxide immediately following its formation further insures both theintegrity of the protective oxide film and its self-limited thickness toa very small dimension only sufficient to its protective function. Theprotective oxide may thus be considered as an intermediate product ofthe etching process in accordance with the invention which is presentonly for an extremely short time and distance ahead of the surface atwhich material is removed.

While not wishing to be held to any particular theory of the chemicalreaction or reactions which may occur during the practice of theinvention, two etching processes suitable to the practice of theinvention will now be described. These processes will serve as guidanceto those skilled in the art in regard to the suitability of otheretchants and etching processes for practice of the invention.

The processes which will now be described are believed to exploit thefact that SSQ materials HSQ (HSiO_(3/2)) and MSQ are deficient in oxygenrelative to the stable silicon dioxide (SiO₂). This deficiency of oxygenis an incident of the polymeric nature of these materials and theprocesses described below are preferred techniques for supplyingadditional oxygen to take part in the chemical reaction to form SiO₂,which is the preferred protective film. However, it should be understoodthat the same basic principles would apply to dielectric materials basedon other Group III-V semiconductor materials and protective films otherthan oxides, such as nitrides. However, other materials may increase thecriticality of process parameters to obtain a suitable result.

In particular, the important factor in choice of processes, processparameters (e.g. reactant concentrations) and etchants is that theetching process must be suitable for both anisotropic etching of the SSQmaterial and the intermediate protective film material at substantiallythe same rate and which is similar to the diffusion rate of the reactantin the SSQ material at the chosen process temperature in order tomaintain the thickness of the protective film 40 to a small dimension.In general, however, the processes which will be described below,including the preferred SSQ materials, are preferred, in large part,because the process parameters are not particularly critical.

In this regard, it has been found by the inventors that practice of theinvention is much more easily controllable in a repeatable manner forHSQ than MSQ although similar results are readily obtained using eitherSSQ material. MSQ has been found to be much more reactive underconditions of the preferred processes and hence somewhat lesscontrollable and more critical in regard to etching process time.Therefore, practice of the invention using HSQ is much preferred.

It should be appreciated however, that the preferred processes andalternatives thereto involve reaction(s) under conditions providing anexcess of a reactant material which is concurrently removed by theetching process. That is, the reaction in accordance with the inventionis substantially contrary to a process driven by relative concentrationof materials at the surface being etched, as is often the case inetching processes. Therefore, the effects of the process in accordancewith the invention is highly counter-intuitive and the meritoriouseffects thereof are unexpected.

Specifically, a first preferred process involves a resist strip using anoxygen-containing plasma which removes SSQ material as the pattern ofresist remaining after development is removed. This process thus(externally) provides oxygen ions at the surface of the SSQ material asit is exposed and oxidizes the SSQ material for about 500 Angstromsbelow the exposed surface to form a very thin layer of SiO₂. A similareffect can be achieved in a second preferred process using a reactiveion etch containing fluorocarbons which is believed to liberate oxygenfrom the SSQ material as it is removed. Both processes form a film 40 ofSiO₂ which is sufficient to provide cohesion protection (e.g. againstthe breaking of bonds to anisotropically remove material, avoidance ofcracking, and other like forms of damage to the integrity of thepolymeric structure) of the SSQ material due to the action of thedeveloper since the oxide is substantially impervious to developersotherwise known to be suitable for lithographic processing ofsemiconductor materials.

Thus, as shown in FIG. 5, when the via resist 50 is developed to formaperture 52, the SSQ material 14 is protected from the action of thedeveloper in region 40′ by the surface oxide formed by oxidation andwithout the addition of a further layer or requiring process steps to doso. The via pattern can thus be established by completion of patterningof the resist while completely avoiding any effect on the SSQ material.

As illustrated in FIG. 6, the via can then be anisotropically etched byany desired process to accurately form an opening to a desired structureon the chip without distortion as shown at 27 of FIG. 3, followed bystripping of the via resist. The via aspect ratio is determined, for anygiven transverse dimension of the via, solely by the SSQ layer thicknesswhich can be readily controlled as desired. A blanket layer of metalwill readily fill the via apertures as well as connection recesses 22,described above in connection with FIG. 2. The blanket metal layer maythen be planarized by any known process to complete the connection andvia structure 60 in accordance with the dual Damascene process. Theplanarization process can be halted at or carried through any portion ofthin oxide layer 16 or the thin oxide layer can be removed entirely.

In view of the foregoing, it is seen that the invention provides a dualDamascene process and structure compatible with low dielectric constantSSQ dielectrics. The invention thus provides a structure and methodologyby which robust conductors can be placed in increased proximity withoutincrease of capacitive coupling, thus supporting increased integrationdensity, speed of signal propagation and noise immunity. The process canbe carried out and the structure formed without additional process stepsbeyond a process in which manufacturing yield would be greatlycompromised and at reduced cost relative to processes which would havelower manufacturing yields and result in an inferior structure. Beyondthe applications to flowable oxide materials and resist developerinteractions discussed above, the method of the invention can be appliedto provide a protective surface covering which is impervious to many orat least selected materials when no other form of protection ispractically or economically available. The process in accordance withthe invention can be repeated at will to form multiple dual Damascenelayer constructions or to perform three or more separate patternings ofa single layer or a combination thereof.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A method of forming asemiconductor device including steps of forming a pattern of resist on asurface of a material, removing a portion of said material in accordancewith said pattern of resist while forming an intermediate material at anexposed surface of said material as said material is removed, saidintermediate material being substantially impervious to a selectedmaterial, forming a further pattern of resist on a surface of saidintermediate material, and removing a portion of said intermediatematerial and a further portion of said material in accordance with saidfurther pattern of resist.
 2. A method of forming a semiconductor deviceas recited in claim 1, wherein said step of removing a portion of saidmaterial includes resist stripping with an oxygen plasma.
 3. A method offorming a semiconductor device as recited in claim 1, wherein said stepof removing a portion of said material includes reactive ion etchingwith a fluorocarbon.
 4. A method of forming a semiconductor device asrecited in claim 1, wherein said material is a silsesquioxane material.5. A method of forming a semiconductor device as recited in claim 4,wherein said step of removing a portion of said material includes resiststripping with an oxygen plasma.
 6. A method of forming a semiconductordevice as recited in claim 4, wherein said step of removing a portion ofsaid material includes reactive ion etching with a fluorocarbon.
 7. Amethod of forming a semiconductor device as recited in claim 1, whereinsaid material is a material selected from the group including hydrogensilsesquioxane and methyl silsesquioxane.
 8. A method of forming asemiconductor device as recited in claim 5, wherein said step ofremoving the portion of said material includes resist stripping with anoxygen plasma.
 9. A method of forming a semiconductor device as recitedin claim 5, wherein said step of removing a portion of said materialincludes reactive ion etching with a fluorocarbon.
 10. A semiconductordevice formed by a process including steps of forming a pattern ofresist on a surface of a material, removing a portion of said materialin accordance with said pattern of resist while forming an intermediatematerial at an exposed surface of said material as said material isremoved, said intermediate material being substantially impervious to aselected material, forming a further pattern of resist on a surface ofsaid intermediate material, and removing a portion of said intermediatematerial and a further portion of said material in accordance with saidfurther pattern of resist.
 11. A semiconductor device formed by aprocess as recited in claim 10, wherein said step of removing a portionof said material includes resist stripping with an oxygen plasma.
 12. Asemiconductor device formed by a process as recited in claim 10 whereinsaid step of removing a portion of said material includes reactive ionetching with a fluorocarbon.
 13. A semiconductor device formed by aprocess as recited in claim 10, wherein said material is asilsesquioxane material.
 14. A semiconductor device formed by a processas recited in claim 13, wherein said step of removing a portion of saidmaterial includes resist stripping with an oxygen plasma.
 15. Asemiconductor device formed by a process as recited in claim 13, whereinsaid step of removing a portion of said material includes reactive ionetching with a fluorocarbon.
 16. A semiconductor device formed by aprocess as recited in claim 13, wherein said flowable oxide material isa material selected from the group including hydrogen silsesquioxane andmethyl silsesquioxane.
 17. A semiconductor device formed by a process asrecited in claim 14, wherein said step of removing a portion of saidmaterial includes resist stripping with an oxygen plasma.
 18. Asemiconductor device formed by a process as recited in claim 14, whereinsaid step of removing a portion of said material includes reactive ionetching with a fluorocarbon.